1. Field of the Invention
The present invention relates to an organic thin film element.
2. Description of the Related Art
The research has recently been activated to realize elements having novel device functions by using the artificially controlled arrangements of various organic thin films. It is studied to find new photoelectric properties and their applications as electronic and optical elements having sophisticated multifunctionalities, which is required by the future information technology. For example, the studies of organic electroluminescence elements using organic thin films formed by vacuum deposition, and the studies of MIS elements using organic thin films as an insulator formed by the Langmuir-Brodgett technique (hereafter referred to as the LB technique), are well known. However, the research has just begun in view of the evaluation of physicochemical properties of organic films with the thickness less than 1000.ANG. and there are only a few studied examples compared with those of organic materials in bulk phase. Therefore, elements having novel functions and effectively utilizing the properties of organic films have not been realized at present.
From the viewpoint of the application of organic thin films to elements, much attention has been paid to an intermolecular charge transfer phenomenon observed for organic solids. The organic material called a charge-transfer complex includes both donor molecule (D molecule) having a low ionization potential and acceptor molecule (A molecule) having a high electron affinity. These compounds have strong molecular interactions between doner and acceptor molecules, giving rise to a charge-transfered state. For example, a complex consisting of perylene (D molecule) and tetracyanoquinodimethane (TCNQ, A molecule) has a degree of charge transfer of 0.5 or less (neutral). On the other hand, a complex consisting of tetramethylphenylene diamine (TMPD) and TCNQ has a degree of charge transfer of 0.6 or more (ionic) in which each molecule becomes partly positive or negative. Also, a complex consisting of tetrathiafulvalene (TTF and chlorani (CA) is known in which a neutral-ionic phase transition is caused due to a change in temperature or pressure and nonlinear electric response is observed (J. B. Torrance et al.: Phys. Rev. Lett., 46, 253 (1981)).
The reasons for much attention being paid to the charge-transfer complexes are that various combinations between D and A molecules are utilized molecular functions caused by specific charge transfer interactions can be easily designed, and the functions are freely controlled by external energy such as electric field and light.
When the charge-transfer complex is applied to elements, it is important that neutral-ionic transition can easily be caused by an electric field or light. The following are considered as the conditions when the neutral-ionic transition phenomenon is caused by electric field. (A) The difference of energy between the ground state i.e. the neutral state and the excited state i.e. the ionic state should be small. (B) The element structure should be realized so that high electric field can be applied to the charge-transfer complex. To realize the element to meet the condition (B), the following measures are adopted; use of a thin film of the charge-transfer complex not a bulk crystal, forming of an insulating layer between an electrode and the charge-transfer complex so that no current flows through the thin film of the charge-transfer complex, and use of an insulating film with a high relative permitivity.
For measures to realize the condition (A), it is considered to select molecules most suitable for the complex. However, it is difficult to actually realize the condition (A). Discussion for the condition (A) is described in more detail below. The threshold electric field E.sub.th where neutral-ionic transition occurs is determined by the following equation. EQU E.sub.I -E.sub.N =eE.sub.th d
where,
e: elementary electric charge, PA0 E.sub.I : energy for an ionic state of D.sup.+ -A.sup.-, PA0 E.sub.N : energy for a neutral state of D-A, PA0 D: distance between D and A molecules, approximately ranging from 3.0 to 3.5.ANG.
From the equation, it is found that the smaller the energy difference between the neutral and ionic states the lower the threshold electric field E.sub.th. In this case, E.sub.th should be lower than breakdown field strength of the charge-transfer complex film or that of the insulating layer. It is actually preferable that the value is kept at 2 to 3.times.10.sup.6 V/cm or less. Therefore, it is preferable that the value meets the following inequality. EQU E.sub.I -E.sub.N &lt;0.1 eV
The charge-transfer complex having a small value of "E.sub.I -E.sub.N " includes one in which transition from neutral to ionic state is caused due to application of pressure or decrease of temperature and the other of which charge-transfer absorption band is present at the wave-length of 0.8 .mu.m or more. Specifically, the following charge-transfer complexes are listed: PTZ-TCNQ, TMDAP-TCNQ, TTF-CA, TTF-fluoranil, dibenzoTTF-TCNQ, DEDMTSeF-dimethylTCNQ, TMDAP-fluoranil, TTF-dichlorobenzoquinone, perylene-tetrafluoroTCNQ, peryene-DDQ, perylene-TCNE, perylene-TCNQ, and perylene-fluoranil.
However, among the above charge transfer complexes only for PTZ-TCNQ and TTF-CA the value "E.sub.I -E.sub.N " is approximated to 0.1 eV though it exceeds a little. Therefore, a considerably high electric field of 10.sup.6 V/cm or more should be required.